In recent years, an advancement in technologies of portable electronic devices is remarkable and electronic devices such as portable telephones and note book computers begin to be acknowledged as fundamental technologies supporting an advanced information society. Research and development of high-functional electronic devices are forwarded energetically and the consumption power of the electronic devices is proportionally increasing. On the other hand, the electronic devices are demanded to operate for a longer time. Accordingly, higher energy density of secondary batteries that are a driving power source is necessarily desired. Furthermore, extension of the cycle life as well is desired from the viewpoint of environmental consideration.
The energy density of a battery is desirable to be higher from the viewpoint of an occupied volume and a weight of a battery incorporated in an electronic device. At the present time, almost all devices incorporate a lithium ion secondary battery because the lithium ion secondary battery has better energy density at high voltages than other battery systems.
Usually, a lithium ion secondary battery uses a lithium transition metal composite oxide such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2) in a cathode and a carbon material in an anode and is used at an operation voltage in the range of 4.2 to 2.5 V. A reason why a terminal voltage is raised up to 4.2 V in a unit cell largely depends on excellent electrochemical stability of a non-aqueous electrolyte material and a separator.
In order to make the lithium ion secondary batteries higher in the performance and to expand applications thereof, much research is being performed. As one of the researches, a method of raising a charge voltage is under study. According to the method, the energy density of a cathode active material including lithium cobalt oxide is raised and thereby a lithium ion secondary battery is made larger in capacity.
However, when charge and discharge are repeated at high capacity, there is a problem in that the capacity is deteriorated and thereby a battery lifetime is shortened. Furthermore, when a battery is used under high temperature environment, gas is generated inside of the battery and thereby problems such as leakage and battery deformation are caused.
In this connection, for instance, Japanese Patent No. 3172388 shown below discloses a method where a surface of a cathode is covered with a metal oxide so as to improve the battery characteristics such as the charge-discharge cycle characteristics. Furthermore, Japanese Patent No. 3691279 shown below discloses a method where a surface of a cathode active material is covered with a metal oxide so as to heighten structural stability and thermal stability.
In the surface coating of a cathode active material, effects of improvements in the cycle characteristics or thermal stability owing to the coating forms are being studied as well. For instance, in Japanese Patent Application Laid-Open (JP-A) Nos. 7-235292, 2000-149950, 2000-156227, 2000-164214, 2000-195517 and 2002-231227 shown below, methods of uniformly coating a lithium transition metal composite oxide are disclosed. Furthermore, in JP-A No. 2001-256979 shown below, a cathode active material where a block of metal oxide is attached onto a metal oxide layer is disclosed.
Elements used in the surface coating as well are studied. For instance, JP-A No. 2002-164053 shown below discloses a cathode active material in which at least one surface-treated layer containing at least two coating elements is coated on a surface of a lithium compound that is a core.
Japanese Patent No. 3054829 shown below discloses a battery having excellent charge-discharge characteristics, which uses a material of which a particle surface is covered with phosphorus (P). JP-A No. 05-36411 shown below discloses a battery having excellent charge-discharge cycle characteristics and large current charge-discharge characteristics, which uses a cathode to which phosphorus (P) is added. Furthermore, Japanese Patent No. 3192855 shown below discloses a method of forming a layer containing boron (B), phosphorus (P) or nitrogen (N). Still furthermore, JP-A Nos. 10-154532, 10-241681 and 11-204145 shown below disclose a method of letting a phosphate compound contain in a cathode.
However, the coating element, coating method and coating form, which are disclosed in Japanese Patent Nos. 3172388 and 3691279 disturb diffusion of lithium ions. Accordingly, there is a disadvantage in that a sufficient capacity is not obtained in a charge-discharge current value in a practical region.
According to the methods disclosed in JP-A Nos. 7-235292, 2000-149950, 2000-156227, 2000-164214, 2000-195517 and 2002-231227, although a high capacity is maintained, it is insufficient to largely improve the cycle characteristics and to suppress the gas from generating. Furthermore, when a cathode active material having a structure where a block of a metal oxide is attached onto a metal oxide layer is prepared according to the method disclosed in JP-A No. 2001-256979, sufficient charge-discharge efficiency is not obtained and a capacity results in largely deteriorating.
The advantage of JP-A No. 2002-164053 is limited only to an improvement in the thermal stability. Furthermore, when a cathode active material is prepared according to a producing method disclosed in JP-A No. 2002-164053, a uniform multilayered layer is formed, and, in particular, a gas is not suppressed from generating but the gas generation is increased on the contrary.
Japanese Patent No. 3054829, JP-A No. 05-36411 and Japanese Patent No. 3192855 intend to improve the cycle characteristics by adding or coating phosphorus to or on the cathode active material. However, the technologies where only an inactive light element is applied to lithium are incapable of obtaining a sufficient reversible capacity.
JP-A No. 10-154532 discloses a technology relating to safeness at the time of overcharging. Furthermore, only by simply mixing a phosphate compound in a cathode, in actuality, a sufficient advantage is not obtained. Similarly, in JP-A Nos. 10-241681 and 11-204145 as well, a phosphate compound is simply mixed in a cathode. Accordingly, a sufficient advantage is not obtained.
As mentioned above, when a cathode active material is modified, the cycle characteristics or thermal stability are improved to some extent, however, a battery capacity tends to be lowered on the other hand. Furthermore, an extent of an improvement in the battery characteristics, which is obtained by the methods described above, is not sufficient and the suppression of gas generated inside of a battery under a high temperature environment is demanded to be further improved.